Two similar systems for electromagnetic flow measurements are described in British Patent Nos. 2,003,274 and 2,056,691. In both of the systems disclosed therein, the flows whose difference is to be measured are flowed through two parallel conduits. In accordance with British Patent No. 2,003,274, such measurement is effected by means of three electrodes, which are influenced by both the flowing medium and an external magnetic field. In accordance with British Patent No. 2,056,691, two main electrodes are influenced in the same manner, and they cooperate with a plurality of earth, or grounded, electrodes.
Both of these systems suffer, however, from the disadvantage that the flowing media can influence the two conduits, and the electrodes themselves in different ways. If, for example, these devices are used for measuring ultrafiltration during dialysis, deposits from the impure water are obtained on the electrodes and/or on the conduit walls themselves. A deposit of the order of few .mu.m, or even less, can then result in considerable reading errors.
An improvement to these two systems is described in European Patent No. 106,940, in which possible reading errors are compensated for by means of repeated calibration of the device.